Gas turbine engines have control systems which determine various operational settings of the engine. For example, scheduling algorithms can be used to adjust the fuel flow and the angular positions of variable-pitch stator vanes (VSVs), and other control inputs to meet safety criteria (avoidance of compressor surge, lean blow-out etc.) and power and efficiency objectives. The control systems typically receive as inputs operating parameters and settings that, in conjunction with such scheduling algorithms, determine turbine control settings to achieve a desired operation. Measured input operating parameters may include compressor inlet pressure and temperature, compressor exit pressure and temperature, turbine exhaust temperature, and engine power output. The schedules relate operational quantities, such as a temperature to a pressure ratio or a temperature to a fuel split, and the output of the schedules thereby determines the appropriate adjustment of control system command inputs to the engine, such as fuel flow, fuel split, and vane angular position.
A problem arises, however, that such schedules are typically designed around a fully deteriorated, worst case engine, but the schedules are used even when the engine is brand new. In addition, such schedules do not take account that no two engines are the same, due to e.g. hardware variations from manufacturing tolerances, and/or hardware modifications during service.
A further problem is that while such an approach allows the engine to be operated in a manner consistent with, for example, safety criteria and power and efficiency objectives, there may be other engine considerations which are compromised or overlooked. For example, it is desirable that an engine has a long time-on-wing through a reduced requirement for maintenance shop visits. One cause of such visits is transient overshoot. In particular, during a take-off slam acceleration, shaft speeds and the turbine entry gas temperature may temporarily exceed agreed amounts of margin between respective design limits and maximum achievable values in service. This is illustrated in FIG. 1, which shows a plot of normalised turbine gas temperature overshoot against time for such slam acceleration from low idle. During the initial phase of the transient, the gas temperature briefly reaches 96% and then drops to 92% of the final stabilised value, indicated on the figure as the “fast transient overshoot”, generating rapid temperature gradients in the hot end of the turbomachinery with detrimental effects on component life, risk of turbine blading rubbing against the liner excessively, and risk to the integrity of thermal barrier coatings if present. The fast transient overshoot is then followed by a slow transient overshoot which can enhance component degradation and necessitate an early shop visit.
Similar overshoot problems may apply to control of other gas turbine engine components, such as active casing control of blade tip clearances by varying the amount of a compressed air bleed used to cool the casing. In this case, the controlled component can be a compressed air bleed valve.
It would be desirable to effect an engine operation that, while meeting safety criteria and power and efficiency objectives, is also able to reduce or avoid the incidence of engine overshoots during transients.